Magnetic resonance imaging systems acquire diagnostic images without relying on ionizing radiation. Instead, MRI employs strong, static magnetic fields, radio-frequency (RF) pulses of energy, and time varying magnetic field gradient waveforms. Thus, MRI is a non-invasive procedure for producing internal pictures of a subject. Two or three-dimensional diagnostic image data is acquired for respective “slices” of a subject area. These data slices typically provide structural detail having, for example, a resolution of one millimeter or better.
Programmed steps for collecting data that is then used to generate the slices of the diagnostic image are known as a magnetic resonance (MR) image pulse sequence. The MR image pulse sequence includes generating magnetic field gradient waveforms applied along up to three axes, and one or more RF pulses of energy. The set of gradient waveforms and RF pulses are repeated a number of times to collect sufficient data to reconstruct the image slices.
Data is acquired during respective excitations of an MR device. Ideally, there is little or no variation in the nuclear magnetization during the respective excitations. However, movement of the subject caused, for example, by breathing, cardiac pulsation, blood pulsation, and/or voluntary movement, may change the nuclear magnetization from one excitation to the next. Nuclear magnetization changes may degrade the quality of the MR data used to produce the images
Acquiring an MRI image takes a period of time. The period of time is determined, at least in part, by the number of scans that are taken and the number of data acquisitions in each scan. If the object being imaged moves during the scan then artifacts can be introduced into the image. Very small motions (e.g., 1 mm, 1° of rotation) can introduce artifacts like blurring and ghosting. Some patients may have difficulties lying completely still, which can lead to MRI images of these patients being degraded by a rotational motion. Furthermore, some types of motion (e.g., heartbeat, respiration) require additional technologies for reducing the effects on imagery. Since these technologies may not yield ideal results, they too can lead to the degradation of MRI images due to rotational motion.